I suspect that blow-overs are a bit of an occupational hazard in sportscar racing, on low downforce circuits in particular. A Porsche 911 GT1 suffered an almost identical accident at Road Atlanta in 1999 (I think). The 'yump' in the track, both on the Mulsanne and at RA was thought to unsettle the front suspension (aided by the loss of a front canard, in the case of the Merc), allowing air to get under the front of the cars, which were both worryingly aerofoil shaped in profile. The Wright brothers theories took care of the rest. I'm fairly certain that a Sauber took flight in an early 90's practice session too - no doubt one of our experts can confirm or deny. I think it did a full 360 back-flip and ended up back on it's wheels.
The most troubling incidents on the straight in recent times seem to have stayed on the track. The Nimrod collision in 84 (or was it 85?) involving Olson and Salmon was very, very scary though neither car broke the surly bonds of earth. Both La Fosse and Gartner only became airboune after colliding with the guardrail, I believe.
I met Peter Dumbreck recently - he was obviously weary of recounting the details of his flight, but he said that the front end lift happened so gradually that he was only aware of it when the steering wheel stopped working!
